1. Field of the Invention
The present invention relates to a radar device for vehicle used for measuring, for example, following-on distances.
2. Discussion of Background
A FM-CW radar device as illustrated in FIG. 11 is known as a radar device for vehicle, wherein the radar device is miniaturized using an antenna for transmitting and receiving to improve applicability to a vehicle. In FIG. 11, numerical reference 1 designates an oscillator; numerical reference 2 designates a power divider; numerical reference 3 designates a transmitting amplifier; numerical reference 4 designates a circulator; numerical reference 5 designates a transmitting and receiving antenna composed of a hone antenna 51 and a reflecting mirror antenna 52; numerical reference 6 designates a target object; numerical reference 7 designates a receiving amplifier; numerical reference 8 designates a mixer; numerical reference 9 designates a filter; numerical reference 10 designates an automatic gain control (AGC) amplifier; numerical reference 11 designates an A/D converter; numerical reference 12 designates a signal processing unit; numerical reference 13 designates an antenna scanning motor; and numerical reference 14 designates a steering angle sensor. A transmitting means is composed of numerical references 1 through 5; a receiving means is composed of numerical references 4, 5, 7 and 8; a signal processing means is composed of numerical references 9 through 12; and a scanning means is composed of numerical references 13 and 52.
An operation of thus constructed conventional device will be described. The signal processing unit 12 outputs linear voltage signals for frequency modulation. The oscillator 1 generates electromagnetic waves subjected to frequency modulation by the voltage signals for frequency modulation. The electromagnetic waves are divided into two parts by a power divider 2, wherein one of the parts is inputted in the mixer 8. The other part of the electromagnetic waves is amplified by the transmitting amplifier 3. Thereafter, it passes through the circulator 4 and outputted into space from the transmitting and receiving antenna 5. The electromagnetic waves outputted to space from the transmitting and receiving antenna 5 are reflected by the target object 6 and inputted in the transmitting and receiving antenna 5 as receiving electromagnetic waves having a delay time Td with respect to the transmitted electromagnetic waves. Further, when the target object 6 has a relative velocity with respect to the radar device, the receiving electromagnetic waves are inputted in the transmitting and receiving antenna 5 with a Doppler shift fd with respect to the transmitting electromagnetic waves. The electromagnetic waves received by the transmitting and receiving antenna 5 are amplified by the receiving amplifier 7. Thereafter, these electromagnetic waves are mixed with the transmitting electromagnetic waves by the mixer 8 to output beat signals corresponding to the delay time Td and the Doppler shift fd. The obtained beat signals pass through the filter 9 and are inputted in the A/D converter 11 after being amplified in the AGC amplifier 10. The signal processing unit 12 calculates a relative range and a relative velocity for the target object 6 from the beat signals.
In the next, a method of calculating the relative range and the relative velocity will be described. FIG. 12 is an example of calculating a relative range and a relative velocity using the above-mentioned radar device, wherein an ordinate represents a frequency f and an abscissa represents a time t. In FIG. 12, a transmitting electromagnetic wave 100 is subjected to frequency modulation with a frequency bandwidth in sweeping B and a modulation period Tm. The receiving electromagnetic waves 101, 102 has a delay time Td between a reflection of the transmitting electromagnetic wave by the target object 6 existing at a position of a range R and an input in the transmitting and receiving antenna 5. Further, when the target object 6 has a relative velocity, the receiving electromagnetic waves have a Doppler shift of fd with respect to the transmitting electromagnetic waves. At this time, a difference of frequency between a transmitting signal, i.e. the transmitting electromagnetic wave, and a receiving signal, i.e. the receiving electromagnetic waves, in case that a frequency of the receiving signal is increased and a difference of frequency fbd between the transmitting signal and the receiving signal, i.e. the receiving electromagnetic waves, in case that a frequency of the receiving signal is decreased are outputted from the mixer 8 as beat signals. These beat signals are subjected to an analogue-digital conversion by the A/D converter 11, taken in the signal processing unit 12 as data, and subjected to a fast Fourier transformation (FFT) to obtain the above-mentioned fbu and fbd and receiving levels M thereof, wherein the receiving levels of fbu and fbd are usually the same M.
The relative range R and the relative velocity V of the target object is obtainable by the following Equation 1. ##EQU1##
where reference C designates the light velocity of 3.0.times.10.sup.8 m/s; and
reference .lambda. designates a wavelength of carrier wave, wherein .lambda.=4.0.times.10.sup.-3 m when a fundamental frequency of the carrier wave is f.sub.0 =77 GHz.
Incidentally, in case that a plurality of target objects exists, fbu and fbd of an identical target object is selected among a plurality of differences of frequency fbu between the transmitting signal and the receiving signal in case that the frequency of the receiving signal is increased or among a plurality of differences of frequency fbd between the transmitting signal and the receiving signal in case that the frequency of the receiving signal is decreased, and relative ranges R and relative velocities V respectively for the plurality of target objects are obtained by Equation 1.
In the next, a method of operating a direction of the target object 6 by the signal processing unit 12 using the receiving level M will be described. Conventionally, in operating a direction, typical methods such as a mono pulse method, a sequential roving method, and a conical scanning method are disclosed in, for example, JP-B-7-20016. The sequential roving method will be described. This method equal to a method of measuring angle in an ample range by normalizing a difference of two receiving levels of radar beam having different axes as disclosed in JP-A-7-92258.
After measuring a range, a relative velocity, and a receiving level M1 in a predetermined direction .theta.1, the signal processing unit 12 similarly measures a range, a relative velocity, and a relative velocity M2 in a next direction .theta.2 by operating the motor 13. Among these plurality of detected data, the same data of the ranges and the relative velocities are selected to measure an angle based on a relationship of magnitude between the receiving level M1 and the receiving level M2.
Specifically, a sum pattern S(.theta.) and a difference pattern D(.theta.) are obtained from antenna beam patterns B1(.theta.) and B2(.theta.) in the predetermined two directions .theta.1 and .theta.2 as follows: EQU S(.theta.)=B1(.theta.)+B2(.theta.) Equation 4 EQU D(.theta.)=B1(.theta.)-B2(.theta.) Equation 5
In the next, a discriminatior DS(.theta.) standardized by S(.theta.) is obtained. EQU DS(.theta.)=D(.theta.)/S(.theta.) Equation 6
where within a half bandwidth .theta.s of S(.theta.) is monotonously increased or decreased.
By defining a center between the predetermined two directions .theta.1 and .theta.2 as .theta.o, and the half bandwidth of S(.theta.) as .theta.s, an inclination k of DS(.theta.) at around an angle .theta.n standardized by .theta.s is obtained for a case that .theta.n=0.
.theta.n=(.theta.-.theta.o)/.theta.s Equation 7 EQU k=DS(.theta.)/.theta.n Equation 8
Further, DS is obtained by observing the receiving levels M1 and M2 using Equation 9. EQU DS=(M1-M2)/(M1+M2) Equation 9
Resultantly, a direction .theta. of a target object is obtained by Equation 10 using .theta.s, k, and .theta.o which can be previously calculated and the observed DS. EQU .theta.=.theta.s/k.multidot.DS+.theta.o Equation 10
Based on thus measured range, relative velocity, and direction of the target object and a curvature of road obtained from the steering angle sensor 14, it is judged whether or not the target object is a preceding vehicle running in a lane where an own vehicle runs to effect an alarm to a following-on distance, a following-on drive with a safe following-on distance, and so on.
However, when the angle is measured by the above-mentioned Equations, it is not possible to measure angles by comparing receiving levels in respective directions if the AGC amplifier 10 is set to have same gains in at least two of the directions. Meanwhile, when the AGC amplifier 10 is set to have same gains in at least two of the directions, there was a problem in operating a system that, under a situation illustrated in FIG. 13, because reflection intensity of a truck closely positioned in a right lane is large, the AGC amplifier is limited by a level of the truck and a preceding vehicle running in the same lane, which is actually required to be detected, can not be detected.